The present invention relates to a receptacle having at least one electrical outlet, and more particularly, is directed to an electrical outlet that senses the ambient temperature, the receptacle temperature and the temperature of a prong of an electrical plug inserted into the outlet, and that automatically shuts off when any of these temperatures is too hot, and has a reset button for resuming operation.
Many fires are believed to be caused by overloaded electrical outlets, that is, outlets operated with more power transfer than the outlet was designed for. Fires are sometimes caused by a loose connection, a glowing connection and/or a high resistance path. A glowing connection occurs when copper oxide is formed between a copper wire and a steel screw in a small air gap creating carbon which glows.
The condition of too much power usage is always accompanied by increased temperature in at least one of the ambient temperature, the receptacle temperature and the temperature of a prong of an electrical plug inserted into the receptacle, collectively referred to herein as “operating temperature”. To avoid fires, it is desirable for the outlet to sense when the operating temperature is too hot, and to cease operation.
Bimetallic switches are electromechanical thermal sensors. The bimetallic or bi-metal portion consists of two different metals bonded together such as brass and Invar. The metals expand at different rates as they warm, causing the element to twist or curve. The changing geometry is used to make or break an electrical contact. Once temperature has returned to normal levels, they revert back to their original geometry.
For a bi-metal comprising brass and invar, the bending occurs at a metal temperature of about 200° F.; the actual temperature threshold is determined by the design of the bimetal and its materials. The metal can be heated by a loose connection or by ambient air temperature. Typical plastic household wiring insulation and outlet housing melts at a temperature of about 300° F. but operation above 200° F. is not recommended due to its high probability of material distortion.
U.S. Pat. No. 6,166,618 (Robertson) discloses an outlet having a bimetallic dome that interrupts electrical contact when the temperature rises above a predetermined threshold. FIGS. 9 and 10 of the Robertson patent are reproduced as FIGS. 9 and 10 hereof. FIG. 9 shows electrical contacts 76c, 66c. At the bottom of FIG. 9, bimetallic dome 106 is shown in its reset (conducting) state. As the temperature rises above the operating threshold of bimetallic dome 106, it flips from a convex to a concave form. At the top of FIG. 9, there is a bimetallic dome in its tripped (non-conducting state), wherein the section of electrical contact 76c is electrically disconnected from contact member 66c. When the bimetallic dome changes shape, it pushes dielectric rod 110 outwards through hole 108, as shown in FIG. 10. Dielectric rod 110 can be manually depressed to reset the bimetallic dome.
The Robertson configuration has several drawbacks. First, a bimetallic dome is associated with each of the outlets in a duplex receptacle, increasing the cost of the receptacle. Second, the dielectric rod is positioned such that the faceplate of the receptacle must be removed to access the dielectric rod, which is inconvenient. Also, the location of the dielectric rod makes it impossible to quickly see that it has tripped. Third, as the bimetallic dome cools below its operating threshold, it can reset itself back to its original configuration. This automatic resetting can be dangerous to a person working around the outlet; in particular, a worker can be electrocuted by the sudden resumption of current. Fourth, although one outlet of a duplex outlet may be tripped, the other outlet will continue functioning, implying to a casual observer that the first outlet is dead rather than tripped, which could result in worker electrocution.
The Robertson patent also discloses another embodiment, shown in FIG. 11, having dish-shaped bimetallic portions 80 that resets on their own as the operating temperature cools. A reset button is absent.
Thus, there is a need for an outlet that is sensitive to heat and avoids undesirable operation.